(c)
Half-life=6.93 min
$k = \frac{0.693}{6.93} = 0.1$
We know ,$k_{1}$for per cent completion
$k_{1} = \frac{2.303}{t}\log\left( \frac{100}{1} \right)$
$0.1 = \frac{2.303}{t}\log\frac{100}{1}$
$0.1 = \frac{2.303}{t}\log 10^{2}$
$t = \frac{2.303 \times 2}{0.1} = 46.06$